Special Occupancies and the NEC

Find out what agricultural buildings, mobile homes, marinas & boatyards, and temporary installations have in common

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Articles 547 through 590 are the last half dozen Articles in Chapter 5. They cover a wide range of applications within the “Special Occupancies” Chapter of the NEC. In this last revision process, the Code-Making Panels and the Technical Correlating Committee attempted to make the Articles in this chapter of the Code consistent with changes made elsewhere in the document. Remember that Chapters 1 through 4 apply to all installations, and the later Chapters are allowed to supplement and modify the requirements of those four in specific types of applications.

Agricultural Buildings

Two factors have a tremendous influence on the lifespan of agricultural equipment: dust and moisture.

Dust gets into mechanisms and causes premature wear. With electricity on the scene, dust adds two other dangers: fire and explosion. Dust from hay, grain, and fertilizer is highly flammable. Litter materials (e.g., straw) are also highly flammable. Farm animal excrement may cause corrosive vapors that eat at electrical wiring methods and equipment. Thus, Art. 547 includes requirements for dealing with dust and corrosion.

Moisture causes corrosion. Water is present for many reasons, including wash down. Consequently, Art. 547 has requirements for wet and damp environments. Also, moisture can increase the chances of shocks to livestock due to differences in potential. Therefore, Art. 547 requires you to install equipotential planes in concrete floor confinement areas of livestock buildings that contain metallic equipment accessible to animals and are likely to become energized.

Livestock have a low tolerance to small levels of stray electrical current, which can cause loss of milk production and even fatalities. Thus, the NEC contains specific requirements for an equipotential plane in buildings that house livestock. Previous Code editions said Art. 547 applies to areas that:

Have excessive dust, or dust with water, and

Contain a corrosive atmosphere.

Because either condition warrants compliance with Art. 547, the 2011 revision removes the word “and” and replaces it with “or” in 547.1.

GFCI-protected receptacles — The allowance for non-GFCI-protected receptacles was deleted. The 2008 NEC added an allowance for receptacles (for a dedicated load) that aren’t GFCI protected, provided there was a GFCI-protected receptacle within 3 ft of the non-GFCI-protected receptacle.

This baffled many Code users, as this change is the opposite of the changes the 2008 NEC made in 210.8. While exceptions/allowances were being removed from 210.8, Art. 547 seemed to be adding them — without justification. GFCI devices open the circuit when there’s more than 6mA of leakage current. Because listed products should have no more than 0.50mA of leakage (or 0.75mA in certain instances), this exception isn’t necessary.

Luminaires and water — An editorial revision was made to the requirements for luminaires in locations subjected to water. In locations that are exposed to water from condensation or from building cleansing water and solution, luminaires must be of the appropriate type. Previous Code editions addressed this by requiring “watertight” luminaires. By changing the rule to require luminaires that are listed as suitable for use in wet locations [547.8], the rule harmonizes with the general rules of Art. 410.

Equipotential plane — The equipotential bonding described in 547.10 is similar to the requirements for bonding swimming pools in 680.26. Both require an 8 AWG or larger copper conductor, but only 680.26 required this conductor to be solid in the previous Code. Section 547.10 of the 2011 NEC now also requires that the bonding conductor be an 8 AWG or larger solid copper conductor (Fig. 1).

The special bonding requirements of Art. 547 exist because of the sensitivity of livestock to stray voltage/current, especially in wet or damp concrete animal confinement areas. Typically, the voltage difference between metal parts and the earth is too low to present a shock hazard. However, livestock might detect the voltage difference if they touch metal parts. Although potential differences may not be life threatening to the livestock, it’s been reported that as little as 0.50V RMS can affect milk production.

Methods to establish equipotential planes are described in the American Society of Agricultural and Biological Engineers Standard EP473, “Equipotential Planes in Animal Containment Areas.” This standard provides the recommendation of a voltage gradient ramp at the entrances of agricultural buildings.

Mobile Homes, Manufactured Hhomes, and Mobile Home Parks

Among dwelling types, mobile homes have the highest rate of fire. Article 550 addresses some of the causes of those fires. Article 550 recognizes that the same structures used for mobile or manufactured homes are also used for non-dwelling purposes, such as construction offices or clinics [550.4(A)]. Thus, it excludes those structures from the 100A minimum service requirement.

Article 550 differentiates between a mobile home and a manufactured home, providing different requirements for each. For example, you can’t locate service equipment on a mobile home. However, you can install service equipment on a manufactured home (provided you meet seven conditions). Pay close attention to the definitions in 550.2 to understand the NEC distinctions.

Mobile home and manufactured homes aren’t covered by the same building codes as site-built homes. Instead, they’re covered by HUD standards. According to HUD, both are referred to as manufactured homes; the term “mobile home” hasn’t been used for many years. This disparity between the NEC and HUD terminology can cause confusion, so read Art. 550 carefully as you apply it.

The Art. 550 requirements for bathroom receptacles [550.13] are similar to those in 406.9(C). Because Art. 550 doesn’t receive many Code change proposals, its requirements can easily fall out of synch with similar requirements found elsewhere in the NEC. When somebody makes a proposal to the general rules in the rest of the Code, they typically forget to make a proposal to Art. 550.

Section 406.9(C) was recently changed to clarify that not only is a receptacle in a bathtub prohibited, but a receptacle above a bathtub also is prohibited. This change correlates these sections by adding the same text in this rule (Fig. 2).

Similarly, 550.25 has been revised to “catch up” to changes made in 210.12.

Section 550.25(A) now says AFCIs are defined in Art. 100.

Section 550.25(B) now requires AFCIs not only in bedrooms, but also in all other areas required by 210.12(B), including family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, and hallways.

Marinas and Boatyards

Ocean tides rise and fall, while lakes/rivers vary in depth in response to rain. To provide power to a marina or boatyard, you must allow for these variations in water level between the point of use and power source.

Article 555 begins with the concept of the electrical datum plane. Think of it as a line that marks the beginning of a zone where you simply don’t place electrical equipment. Once you determine where this plane is, don’t locate transformers, connections, or receptacles below that line.

Ground fault protection of main feeder — After being presented with evidence of more than 50 deaths and 30 injuries from electrical incidents in marinas and boatyards, the technical committee added a requirement for equipment ground fault protection. At first glance, it seems odd that “equipment” level protection instead of “personnel” protection is the requirement [555.3]. A “Class A” GFCI device (for personnel) opens the circuit when the current to ground is between 4mA and 6mA, while a GFPE (for equipment) trips somewhere between 30mA and 100mA. While this value is too high to directly protect a human from a fatal electric shock, it will open the circuit under most fault conditions. Removing this fault current will, in turn, protect humans against electrical shock.

Temporary Installations

Temporary wiring represents a different standard from normal wiring, but the same rules of workmanship, ampacity, and overcurrent protection apply. It’s just as important to protect construction workers as it is to protect the tenants who will occupy the completed facility.

Temporary wiring must comply with all provisions of the Code, except where specifically addressed in Art. 590 [590.2(A)]. Because there are no rules in Art. 590 that address receptacles in wet locations, these receptacles must comply with 406.9. The 2011 NEC clarifies this requirement [590.4(D)(2)].

All 15A and 20A receptacles installed in a wet location must be within an enclosure that’s weatherproof when an attachment plug is inserted. For other than one- or two-family dwellings, the outlet box hood must be listed for “extra-duty” use if supported by grade. Nonlocking-type 15A and 20A, 125V and 250V receptacles in a wet location must be listed as weather resistant [406.9(B)(1)], as shown in Fig. 3.

Exposed plastic surface material of weather-resistant receptacles must have UV resistance to prevent deterioration from sunlight or ensure it’s minimal. In testing, receptacles are subjected to temperature cycling from very cold to very warm conditions, and then subjected to additional dielectric testing. The rapid transition from cold to warm temperatures changes the relative humidity and moisture content on the device, and the dielectric test ensures this won’t create a breakdown of the insulation properties.

GFCI for personnel — The 2011 Code requires all 15kW or smaller generators to have GFCI protection for the 125V or 125/250V, single-phase, 15A, 20A, and 30A receptacles installed on them. For generators manufactured or remanufactured prior to Jan. 1, 2011, cord sets or other devices that provide portable GFCI protection may be used [590.6(A)(3)]. Section 590.6 was also rewritten using a list format for easier reading and understandability.

Holt is the owner of Mike Holt Enterprises, Inc., Leesburg, Fla. He can be reached at www.mikeholt.com.

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